This invention relates to an air generation unit (AGU) suitable for an aircraft, and more particularly, the invention relates to a pack and a half air generation unit configuration utilizing two air cycle machines (ACM) with each preferably having two turbines.
AGUs typically include at least one ACM and at least one heat exchanger that receives air from a pressurized air source, such as bleed air from an engine, to provide cooled air to the aircraft cabin and cockpit. The AGUs may be packaged within the wings and/or tail section of the aircraft. Accordingly, it is desirable to provide an AGU having a very small package to limit the amount of aircraft structure that must be removed to accommodate the AGU. Furthermore, the AGU must provide sufficient cooling for the size of the aircraft.
A pack and a half AGU has been developed for use in a first prior art system in which two ACMs share a common heat exchanger. The pack and a half configuration provides increased cooling and smaller packaging than two separate AGUs each having their own heat exchanger. The system utilizes a three wheel air cycle machine having a fan, a compressor, and a single turbine.
The prior art AGU includes a heat exchanger having primary and secondary heat exchangers. Bleed air is taken from an intermediate or high pressure stage of a turbine engine. The bleed air is pre-cooled within the primary heat exchanger with the heat being rejected to ram air and then communicated to the compressor of the ACM. After compression, the air is communicated through a secondary heat exchanger to a condenser. Condensed water vapor is extracted by a water collector, and the dehumidifier air is sent to turbine where the air is expanded to generate cold air. The cold air is sent to a mixer and to a distribution system in the aircraft.
A second prior art system utilizes AGUs with a single ACM and single heat exchanger. The system is a four wheel configuration that includes a fan, a compressor, and first and second turbines. The operation of the AGU is similar to the first prior art system AGU, but the second turbine receives the cold air from the first turbine and further expands the cold air to produce subfreezing air. The second turbine produces air sufficient to cool larger aircrafts.
Very large commercial aircrafts are being developed capable of carrying up to 1,000 passengers or more. These large aircraft require AGUs capable of producing very cold temperatures. However, the design constraints for the aircraft require very small packaging with very high reliability. The first prior art system AGU configuration produces conditioned air that is not sufficiently cold for such a large aircraft. The second prior art system AGU configuration provides sufficiently cold air, however, up to four or more AGUs would be required for such a large aircraft, which would necessitate removing significant aircraft structure and would significantly increase weight.
The pack and a half configuration of the first prior art system utilizes solenoid valves external to the manifold to control the flow of air from the turbines to the manifold, which distributes air to and from the condenser. In the event that one of the ACMs is not needed or is malfunctioning, the unwanted ACM is shut down or idled. A dedicated solenoid valve is closed to prevent leakage from the turbine of the unwanted ACM, which would result in large operating inefficiencies. Accordingly, each turbine in a prior AGU has a solenoid associated with it. Hoses and band clamps connect each of the solenoids to the manifold in the AGU. Moreover, numerous wires are used to connect each of the solenoids to a controller. As a result, with prior art AGUs, the number of components greatly increases as the number of turbines in an AGU increases resulting in higher costs and reduced reliability.
Therefore, what is needed is an improved AGU having a small package, increased reliability, and sufficiently cold air for the needs of a large aircraft.